On January 31, 2013, the EPA promulgated amendments to the National Emission Standards for Hazardous Air Pollutants for Major Source: Industrial, Commercial, and Institutional Boilers and Process Heaters. Environmental groups and industry filed for judicial review of the amended final rule. The court issued its decision on July 29, 2016, vacating the maximum achievable control technology (MACT) standards for all subcategories that were affected by the EPA's exclusion of certain sources from consideration when setting the standards. The court also remanded the carbon monoxide standard to the EPA to adequately explain how carbon monoxide acts as a reasonable surrogate for organic hazardous air pollutants. On September 12, 2016, the EPA petitioned the court asking that the MACT standards be remanded without vacatur. On December 23, 2016, the court granted the EPA’s request. In November 2015, the EPA finalized its decision on certain other issues for which it granted reconsideration. Environmental groups filed a petition for judicial review of the reconsideration action. The court issued its decision on March 16, 2018, remanding for further explanation the revised 130 parts per million carbon monoxide emission limits. This final rule will address the issues that were remanded in the two court decisions.